1. Field of the Invention
The present invention is related to an optical resonator structure which automatically compensates for changes in optical characteristics due to changes in temperature. More particularly, the present invention relates to an optical resonator structure constructed of materials having a predetermined index of thermal expansion such that changes in optical characteristics are compensated for by thermal expansion and contraction of the optical resonator structure.
2. Background of the Invention
Many optical devices are constructed in such a manner that it is critical that the optical elements within the device remain in a precise spacial arrangement. For example, the operation of lasers require that the optical elements remain in a precise location in order to assure that the laser continues to operate correctly. Likewise, in many other types of optical devices, such as microscopes, telescopes, and the like, misalignment of the optics within the device results in reduced performance of the device and the possibility of total failure of the device.
One of the problems that is encountered in modern devices, such as lasers, is that they require a significant amount of energy for their operation. This energy is usually provided in the form of electrical energy, which may then be converted to light and heat energy. For example, in the typical ion laser, atoms are excited by electrical energy to an excited ion state. The excited atoms, being inherently unstable have a tendency to return to a lower energy state. Accordingly, photons are emitted by the electrically excited atoms in order for those atoms to return to a lower energy state. During this process both heat and light energy are produced The heat must be dissipated but may be conducted off to surrounding optical components changing their index of refraction. At the same time, heating causes expansion of the various mechanical components of the laser structure resulting in misalignment of the laser.
During operation of an ion laser, the photons emitted during an induced transition of the type mentioned above have the same phase and direction as the inducing wave (i.e., they are coherent with the wave that induces the transition). A single atom may radiate a photon in any direction. However, many atoms distributed over a finite volume and radiating coherently cooperate to generate a wave having the same propagation vector as the inducing wave, within the limits of a diffraction pattern. That is, they amplify the inducing wave.
Thus, the radiation from induced emission has a spectral distribution identical to that of the inducing radiation. Also it is found that certain types of atoms produce certain specific wavelengths of radiation during the energy transition and emission of photons. For example, argon, a common substance for use in a lasing medium, produces approximately nine distinct wavelengths of radiation The most commonly used wavelengths for laser purposes are at 488 nm and 514 nm.
It will be appreciated that once a distinct wavelength of radiation is isolated, it can be used to produce a lasing action. A laser includes a lasable medium positioned between optical reflectors which reflect the optical radiation of the selected wavelength back and forth through the lasable medium to produce stimulated emission of coherent optical radiation. The reflectors define a path between the optical elements, generally referred to in the art as the laser "optical axis." The optical elements (reflectors) and the means for supporting the lasable medium along the optical axis, combined to form a space which is sometimes referred to as the "optical cavity." Typically, one of the reflectors is partially transmissive and permits some of the coherent optical radiation to escape from the resonant optical cavity to thereby provide an output laser beam of coherent radiation.
It will be appreciated that it is extremely critical that the reflectors be maintained in the precise alignment in order to reflect the coherent light energy and to maintain the lasing effect. Even slight changes in the alignment or optical characteristics of the reflectors can result in serious energy losses, wavelength changes, and other effects detrimental to the operation of the device.
It is, therefore, conventional in the art to provide means for rigidly supporting the optical elements, particularly reflectors, with respect to the remainder of the device, including the lasing medium. A number of designs of such devices are well known and conventional in the art. These structures for mounting the various components of the laser are generally referred to as resonators. Resonators generally include a structure for securely mounting the reflector, the laser medium, and any other necessary and desired components in a desired spacial relationship.
One commonly encountered feature comprises a plurality of metal alloy rods, usually constructed of a material such as INVAR.RTM., with a low coefficient of thermal expansion. INVAR is a nickel-iron alloy. In one formulation, known as INVAR 36, the alloy is comprised of 0.02% carbon, 0.35% manganese, 0.20% silicon, 36.00% nickel, and the remainder comprising iron. These rods are used in order to mount the various components of the laser to the laser resonator and to maintain them in a precise spatial relationship.
As was mentioned above, it is discovered in the operation of lasers that a relatively large quantity of heat is produced by the operation of the laser. In the case of ion lasers, the constant electrical excitement of the laser medium and the resulting discharge of heat energy has the tendency to heat the surrounding environment significantly. Even using materials having low coefficients of thermal expansion it is discovered that the operation of the laser is compromised due to thermal expansion or contraction of the components of the laser.
The effects of changes in temperature on the operation of an optical device such as a laser may be encountered in several different forms. For example, in a resonator comprising two facing and parallel flat reflectors, it is necessary to establish and maintain the reflectors in parallel alignment. Misalignment produces a decrease in the level of output stability.
In some types of lasers it is now preferable to employ prisms. Prisms are often used in place of or in conjunction with the maximum, high or total reflector. As the laser beam travels into the prism and is reflected back out, the prism separates the existing light wavelengths into discernible separate beams. Thus, if it is desirable to produce a laser beam having a precise wavelength of 514 nm, the light beam may be directed into a prism and the prism positioned such that only the 514 nm component of the beam travels back along the optical axis. The other wavelengths produced by excitement of argon are directed away from the optical axis by the operation of the prism.
With prisms, as with all optical components, it is found that even a small change in temperature will result in a significant change in the index of refraction of the prism. With a change in the index of refraction, there is also a change in the angle at which the various separated components of the light beam exit the prism. Thus, there may be a misalignment in that the desired portion of the light beam does not travel directly back along the optimum optical axis. It will be appreciated that even slight changes in the index of refraction can result in a reduction in efficiency of the device and a loss of power.
In order to deal with the various effects of temperature change on the operation of optical devices such as lasers, there have been developed a number of different types of devices. As discussed briefly above, one attempted solution has been to construct the resonator using materials that have very low coefficients of thermal expansion such as INVAR. Using INVAR the resonator can be held in precise alignment through a reasonable range of temperature. However, the use of INVAR does nothing to combat changes in the index of refraction of the various optical element. Thus, even using INVAR rods and components, significant loss of performance is observed.
Another attempted solution is to provide complex mechanisms for maintaining the device at a particular predetermined temperature. One type of solution has been to heat the optical components to a particular temperature that can be maintained even though lasing action is taking place. These devices are cumbersome and expensive.
It is found that even using the often complex and expensive types of devices known in the art, that optical misalignment is still encountered to a significant degree. This is particularly true when the index of refraction of the optical element must be considered, in addition to the linear alignment of the optical axis.
In the case of devices in which optical elements are used in which the index of refraction is critical, the only real means for dealing with temperature induced changes in the index of refraction is to control temperature in the complex ways discussed generally above, or to tilt the optical element in such a manner that the exiting light beam is still traveling along the desired optical axis. Titling the optical element (such as prism) is difficult to achieve without undue operator intervention or complex electro-mechanical adjustment system. In addition, when extensive operator intervention is required it is difficult to maximize the output of the device. Thus, the titling of the optical element may result in less than ideal optical alignment.
Accordingly, it will be appreciated that it would be a major advancement in the art to provide means for adequately compensating for thermally induced changes in the operation of optical equipment. In particular, it would be a major advancement in the art to provide means for compensating for changes in the index of refraction of optical elements over the range of temperatures typically encountered in any particular optical device. It would also be an advancement in the art if this type of compensation could be achieved without the necessity of employing complex and expensive heating and cooling mechanisms. It would also be an advancement in the art to provide for compensation of the effects of thermal changes simply by choosing appropriate materials for the construction of the device.
Such and methods and apparatus are disclosed and claimed herein.